Sera from five of 11 patients with neuropathy associated with IgM paraproteinemia (NAIgMPP) and reactivity against myelin-associated glycoprotein (MAG) also had elevated levels of IgM against sulfatide. Although three patients had anti-sulfatide IgM titers of less than or equal to 1:1000, two patients had titers of greater than or equal to 1:50,000. Absorption of patient serum with sulfatide revealed that anti-MAG IgM paraproteins in two patients with high titer anti-sulfatide IgM crossreacted with sulfatide. Our study is the first to show that some anti-MAG IgM paraproteins crossreact with sulfatide, a major acidic glycolipid of myelin. Moreover, some patients with NAIgMPP have polyclonal anti-sulfatide IgM in addition to anti-MAG IgM paraproteins. Therefore, sulfatide may be a target antigen in some patients with NAIgMPP.